Display panels of the type contemplated by the invention are known as SELF-SCAN panels and are described in many patents and copending applications, including Ser. No. 636,919, filed Dec. 2, 1975, now U.S. Pat. No. 3,995,185 and incorporated herein by reference. These devices all include mercury in their gas filling to minimize cathode sputtering. Usually, the mercury is provided in a breakable or openable capsule which is mounted in the panel. At a predetermined time in the manufacturing process, the mercury capsule is opened to release the mercury into the panel. In panels of this type, a problem arises which is related to the optimum location for the mercury capsule and the provision of means for insuring the dispersion of the mercury throughout the panel. The present invention solves this problem by mounting the mercury capsule in operative relation with channels through which the mercury can disperse.